A string instrument typically makes sound by the playing of the instrument's strings with the sound being projected either acoustically or through electrical amplification (for an acoustic instrument or an electric instrument, respectively). The most common string instrument is the guitar with six strings, which is typically played by strumming or plucking the strings with the right hand while fretting the strings with the left hand. The guitar is a type of chordophone, traditionally constructed from wood and strung with either nylon or steel strings and distinguished from other chordophones by its construction and tuning.
The construction of the body is what characterizes the guitar. Certain kinds of guitars are called “solid body guitars” or “hollow body guitars”, and the type of construction influences the tone, and how the tone is produced. Solid body guitars, such as an electric guitar which is not just “electric”, uses the whole guitar body for vibration so there are several parts that form the tone, not just the strings and the pickups as in a hollow body guitar. A solid body is made out of logs of solid wood that are glued together, or out of laminated wood, or plywood. Plywood guitars are cheaper, but they don't sound as good. Solid body guitars don't feedback that easily.
Hollow body guitars as the name implies have a hollow body that is made out of several parts, like a top, a back, and sides, etc. The neck of the hollow body guitar terminates at the heel and does not enter the guitar body. The sound of a hollow body is usually warmer (or more “natural” than that of a solid body. A disadvantage of hollow bodies is that they feedback more easily if the hollow body is too close to the amplifier.
An acoustic guitar is a guitar that uses only acoustic (as opposed to electronic) means to transmit the strings' vibrational energy to the air in order to produce a sound, and typically involves the use of a sound board (top) and a sound box (hollow body) to amplify the vibrations of the string. The source of sound in an acoustic guitar is the string, which is plucked with the fingers or with a plectrum. The string vibrates at a fundamental frequency but also creates many harmonics at different frequencies. The frequencies produced depend on string length, mass, and tension. The string causes the soundboard and sound box to vibrate, and as these have their own resonances at certain frequencies, the soundboard and sound box amplify some string harmonics more strongly than others, hence affecting the timbre produced by the instrument.
The acoustic guitar's soundboard, or top, also has a strong effect on the loudness of the guitar. No amplification actually occurs in this process, because no external energy is added to increase the loudness of the sound (as would be the case with an electronic amplifier). All the energy is provided by the plucking of the string. But without a soundboard, the string would just “cut” through the air without actually moving it much. The soundboard increases the surface of the vibrating area in a process called mechanical impedance matching. The soundboard can move the air much more easily than the string alone, because it is large and flat. This increases the entire system's energy transfer efficiency, and a much louder sound is emitted.
In addition, the acoustic guitar has a hollow body and an additional coupling and resonance effect increases the efficiency of energy transmission in lower frequencies. The air in a guitar's cavity resonates with the vibrational modes of the string and soundboard. At low frequencies, which depend on the size of the box, the chamber acts like a Helmholtz resonator, increasing or decreasing the volume of the sound again depending on whether the air in the box is moving in phase or out of phase with the strings. When in phase, the sound is increased by about 3 decibels and when in opposing phase, it is decreased about 3 decibels. As a Helmholtz resonator, the air at the opening is vibrating in or out of phase with the air in the box and in or out of phase with the strings. These resonance interactions attenuate or amplify the sound at different frequencies, boosting or damping various harmonic tones. Ultimately, the cavity air vibrations couple to the outside air through the sound hole, or have f holes, like a violin family instrument. This coupling is most efficient because here the impedance matching is perfect: it is air pushing air.
FIG. 1 and FIG. 2 illustrate a typical prior art acoustic guitar 10. The guitar 10 has a headstock 12 that is located at the end of the guitar neck 14 farthest from the body 40. The headstock 12 is fitted with machine heads tuners 16 that adjust the tension of the strings 30, which in turn affects the pitch. The nut 18 is a small strip of bone, plastic, brass, corian, graphite, stainless steel, or other medium-hard material, at the joint where the headstock 12 meets the fretboard 22. The nut 18 has grooves that guide the strings 18 onto the fretboard 22, giving consistent lateral string placement, and is one of the endpoints of the strings' vibrating length. The fretboard 22 is a piece of wood embedded with metal frets 20 that encompasses the top of the neck 14. Pinching a string against the fretboard 22 effectively shortens the vibrating length of the string, producing a higher pitch. A guitar's fretboard 22, tuners 16, and headstock 12 all attached to a long wooden extension, and collectively constitute the guitars neck 14. The wood used to make the fretboard 22 usually differs from the wood in the rest of the neck 14. The bending stress on the neck 14 is considerable, particularly when heavier gauge strings are used, and the ability of the neck 14 to resist bending is important to the guitar's ability to hold a constant pitch during tuning or when strings are fretted. The rigidity of the neck 7 with respect to the body of the guitar 10 is one determinant of a good instrument versus a poor one. The heel 24 is the point at which the neck 14 is either bolted or glued to the body 40 of the guitar 10.
The truss rod is a metal rod that runs along the inside of the neck 14. The truss rod is used to correct changes to the neck's curvature caused by the neck timbers aging, changes in humidity or to compensate for changes in the tension of strings. The tension of the rod and neck assembly is adjusted by a hex nut or an allen-key bolt on the rod, usually located either at the headstock 12, sometimes under a cover, or just inside the body of the guitar underneath the fretboard 22 and accessible through the sound hole 28. Some truss rods can only be accessed by removing the neck 14. The truss rod counteracts the immense amount of tension the strings 30 place on the neck 14, bringing the neck 14 back to a straighter position.
The lack of extension of the neck 14 into the body 40 (see FIG. 2) is a feature that distinguishes a hollow body acoustic guitar or acoustic instrument from a solid body guitar or instrument. Solid body instruments have a neck-through-body construction that extends the piece (or pieces, in a laminate construction) of wood used for the neck through the entire length of the body, essentially making the neck a core of the body, resulting in improved stability. The strings, fretboard, pickups and bridge are all mounted on this piece. In the typical solid body, “ears” or “wings” (i.e., side parts of the body) are glued or laminated to the central “stick”. Neck-thru construction allows easier access to upper frets, because there is no need for a heel 24—the thickened area where a neck would attach to the body. Many musicians assert that neck-through construction provides greater sustain of sound and allows the instrument to stay in tune longer.
The body 40 of the guitar 10 has a cavity contained by the bottom 26, body side wall (rib) 32, and top or soundboard 36. The soundboard 36 has the sound hole 28 positioned under the strings 30. The strings 28 terminate at the bridge 38, which transfers the vibration from the strings 30 to the soundboard 36, which vibrates the air inside of the guitar, thereby amplifying the sound produced by the strings. The saddle 34 is the part of the bridge 38 that physically supports the strings 30. The saddle 34 provides the end point for the string's vibration at the correct location for proper intonation.
FIG. 2 is a prior art sectional view with a portion of the side wall 32 and top 36 of the acoustic guitar 10 removed showing the hollow cavity 42. As can be seen the neck terminates at the heel 24, and does not extend in the body 40 to the tail block 44. A series of braces 46 provide structural support to the body 40.
While acoustic instrument designs have evolved over time, the hollow body design lacks the advantages of solid body instruments. Thus, there exists a need for a hollow body design that incorporates a neck through body design with reduced bracing for enhanced total sound quality.